1. Field of the Invention
This invention relates to a method of and an apparatus for producing a micro lens. This invention further relates to a method of forming a mold (mother) for forming the micro lens.
2. Description of the Related Art
In a solid state image taking device such as a CCD image sensor, a micro lens for increasing light collecting efficiency is provided corresponding to each of a plurality of photodetector elements such as a photodiode. Usually, the micro lenses form a micro lens array comprising a plurality of two-dimensionally arranged micro lenses, and the micro lens is often used in the form of a micro lens array.
Further, the micro lens array is often used in a display and an image exposure system as a spatial light modulator element as well as the solid state image taking device. In Japanese Unexamined Patent Publication No. 2000-332226, an example of the micro lens array of this type is disclosed.
Though there have been proposed various methods of producing the micro lenses, methods in which the surface tension of the lens material is prevailing from the viewpoint of facilitation of formation of curved face of the lens. In Japanese Unexamined Patent Publication No. 59(1984)-147586, there has been disclosed a method in which a thermoplastic resin pattern is formed on an solid state image taking device and a micro lens is formed by fusing the thermoplastic resin pattern, as an example of such methods. Further, in Japanese Unexamined Patent Publication No. 9(1997)-122573, a method in which a low contact angle area and a high contact angle area are formed on a substrate, and a lens is formed by causing a coating solution (the lens material) to selectively adhere to the low contact angle area on the basis of the difference in wettability between the low contact angle area and the high contact angle area.
In the conventional method in which a lens is produced by fusing resin, which forms the lens, as described in Japanese Unexamined Patent Publication No. 59(1984)-147586, positive resist has been used in view of the facilitation of patterning. However, since the resist is based on phenolic novolak and accordingly, is thermosetting, setting also gradually progresses in the temperature range of 130 to 160° C. which is the softening range of the resist. Accordingly, there is difficult to form a curved surface as designed in this method. Especially, in the case of a low lens (for instance, a lens which is not smaller than 10 μm in diameter and is not smaller than 100 μm in radius of curvature), the curved surface can be formed only in the periphery of the pattern and the obtained micro lens does not regularly function since the central portion can be solidified before the fluidity of the resist becomes sufficient.
Further, in the method disclosed in Japanese Unexamined Patent Publication No. 9(1997)-122573, it is necessary to uniformly apply the lens material to substrates which are different in wettability. However it is difficult to uniformly apply the lens material when there is unevenness due to the structure on the under side as in the solid state image taking device and fluctuation in shape can increase from lens to lens.